using System;

public class Program

{

    public static void Main()

    {       

        Console.WriteLine('.'.IsLetter());

        Console.WriteLine('.'.IsLetterOrSeparator());

        int? num=12;

        if(num is not null){

            Console.WriteLine(num);

        }               

    }   

}

​

public static class Ex{

    public static bool IsLetter(this char c) =>c is >= 'a' and <= 'z' or >= 'A' and <= 'Z';

    public static bool IsLetterOrSeparator(this char c) => c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z') or '.' or ',';

}